The consumption of hot water represents a significant portion of national energy consumption and contributes to concerns associated with global climate change. Utilizing heat recovered from the sewer, or the stored heat by utilizing heat pumps with a borehole geothermal energy storage system, are simple and effective ways of heating water for domestic purposes. Reclaiming heat from the waste warm water that is discharged to the sewer or stored heat in a borehole geothermal energy storage system can help reduce natural gas energy consumption as well as the associated energy costs and greenhouse gas emissions. In this paper, sewer waste heat recovery is compared with heat pumps using geothermal energy storage systems for a small community shared water heating system including commercial and institutional buildings. It is found that the sewer heat exchanger method is relatively economical as it has the smallest rate of return on investment for the selected community size. The findings also demonstrate a reduction occurs in natural gas consumption and fewer CO2 gas emissions are emitted to the atmosphere. The results are intended to allow energy technology suppliers to work with communities while accounting appropriately for economic issues and CO2 emissions associated with these energy technologies.

Various heat transfer models are reported for vertical ground heat exchangers, and several basic analytical and numerical models of vertical heat exchangers are described and compared, and recent developments are discussed. To examine the effect of temperature rise in the soil surrounding a vertical ground heat exchanger on the performance of the ground heat pump, the heat transfer model that represents the temperature rise and heat flows outside the borehole is often coupled to the models inside the borehole via the borehole wall temperature. This temperature is an important factor that affects the heat delivery/removal strength of the system to/from the ground. In the current study, the results of a semi-analytical model that couples a model outside the borehole with one inside the borehole taking into account the transient borehole wall temperature is described. The results of this model for a constant borehole wall temperature are compared with those for a transient one with a numerical model. It is shown that transient borehole wall temperature results in more accurate temperatures for the circulating fluid flowing to the heat pump.

The use of geothermal energy systems is widespread but, having had a revival in the 1980\'s and recently, both the sustainability and impact of these systems on the environment are now being questioned. Due to its efficiency, the use of geothermal energy is advantageous in many cases. However, little research is available to guide regulatory agencies and industry towards designs and installations that maximize their sustainability. One potential hindrance to the sustainability of these systems at their design efficiency is the thermal loss from the system itself, which can affect adjacent systems and the surrounding ground. Studies show that interference effects are present in some installed geothermal systems. The influence of these systems on each other implies that they have a spacing that is smaller than the threshold spacing for such systems to avoid thermal interactions, and indicates that there is a limit to the density of geothermal development that can occur in a given region of the ground. Many studies in the area of geothermal energy have focused on modeling single ground boreholes. The potential existence of thermal interaction among multiple boreholes is identified in the literature, but not formulated, and the affecting parameters have not been assessed in detail. In order to model interacting borehole systems, Koohi-Fayegh and Rosen (2011) evaluated the temperature response in the soil surrounding multiple boreholes in a numerical study. They assumed that the heat flux from the borehole wall is constant and, therefore, that heat conduction in the direction of the borehole length is negligible for a major part of the solution domain. In the current study, the assumption of constant heat flux along the borehole wall is examined by coupling the problem to the heat transfer problem inside the borehole. A numerical finite volume method in a three dimensional meshed domain is used to model the conduction of heat in the soil surrounding boreholes. In order to determine the heat flux boundary condition, the analytical quasi-three-dimensional solution to the heat transfer problem of the U-tube configuration inside the borehole (Zeng et al, 2003) is used. This solution takes into account the variation in heating strength along the borehole length due to the temperature variation of the fluid running in the U-tube. Thus, critical depths at which thermal interaction occurs can be determined. References Koohi-Fayegh S., Rosen M. A., Examination of thermal interaction of multiple geothermal storage and heat pump systems, Proc. 3rd International Conference on Applied Energy, 16-18 May 2011, Perugia, Italy, pp. 3473-3486. Zeng, H. Y., N. R. Diao, Z. Fang, 2003, Efficiency of vertical geothermal heat exchangers in ground source heat pump systems. Journal of Thermal Science 12(1):77–81.